1. Field of the Invention
The present invention relates to a wiring board, and specifically to a wiring board including a differential transmission line for transmitting differential signals.
2. Description of the Related Art
With the development of electronic information device, faster signaling has been required in wiring boards for use in electronic equipment. As a method of fast signal transmission, a differential transmission system has been used. The differential transmission refers to transmitting high-frequency signals (e.g., high-frequency range of 20 GHz or more), being opposite in polarity, to a pair of lines, and the signals are evaluated based on the potential difference between the lines. For this reason, the differential transmission system has an advantage of being resistant to common mode noise. Examples of wiring boards employing such a differential transmission system are shown in Japanese Patent Publication No. 2736107 and Japanese Unexamined Patent Application, First Publication No. 2003-31724.
FIG. 9 is a cross-sectional view showing an exemplary configuration of a conventional wiring board.
In a wiring board 100, an optical device 115 is disposed on a face 110a of a board 110. On the board 110, a differential transmission line 113 consisting of two wirings 112A and 112B disposed in parallel are formed. An end of the differential transmission line 113 and the optical device 115 are electrically connected to each other via a joint metal 114. To the other end of the differential transmission line 113, for example, a printed board 118 is connected via a joint metal 117. The optical device 115 includes a plurality of terminals 115b for inputting and outputting electric signals, and the distance (pitch) between the terminals 115b is, for example, approximately 100 μm. On the other hand, input/output terminals are formed on the printed board 118 which is connected to the other end of the differential transmission line 113, and the distance between the terminals is, for example, approximately 400 μm. The differential transmission line 113 serves as a wiring for transmitting differential signals and converts the narrower pitch of the terminals of the optical device 115 to the wider terminal pitch of the printed board 118.
An optical functional part 115a for emitting or receiving light is provided in the optical device 115, the face opposing the board 110. The board 110 is made of, for example, glass or the like, and inputting/outputting of optical signals by the optical device 115 is performed through the board 110.
In the wiring board 100 including the differential transmission line 113, the other end of the wirings 112A and 112B constituting the differential transmission line 113 is formed on an insulation resin layer 111 having a uniform thickness. The insulation resin layer 111 is provided for relaxing the stress which is transmitted from the board 110 to the printed board 118. The connection reliability between the wirings 112A and 112B and the printed board 118 is ensured by the insulation resin layer 111
When inputting/outputting optical signals through the board 110 by the optical device 115 mounted on the board, if the insulation resin layer 111 and the wirings 112A and 112B are formed in the optical path, the transmission of optical signals is inhibited by them. In order to solve the above-described problem, as shown in FIG. 9, the insulation resin layer 111 and wirings 112A and 112B may not be provided in the optical path but the insulation layer 111 may be provided at the other end of the wirings 112A and 112B.
However, the following issues arise in the conventional wiring board 100.
FIGS. 10A-10D are an enlarged view showing the differential transmission line 113 (wirings 112A and 112B) of the conventional wiring board 100. FIG. 10A is a plan view, and FIGS. 10B-10D are a cross-sectional view taken along line segment X4-X4′, line segment X5-X5′ line segment X6-X6′ shown in FIG. 10A, respectively.
As shown in FIGS. 10A-10D, the wirings 112A and 112B constitute the differential transmission line 113 which transmits high-frequency signals. The wirings 112A and 112B are formed so as to straddle (traverse) a stepped portion 120 having a height corresponding to the thickness of the insulation resin layer 111. However, as described above, it is necessary to convert the narrower pitch of the terminals of the optical device 115 to the wider pitch of the printed board 118. In a case where the extending direction of the differential transmission line 113 and the extending direction of the linear periphery 111b of the insulation resin layer 111 obliquely intersect each other in a plan view of the wiring board 100, in order to connect one end and the other end of the wirings with the shortest distance while meeting the above-described requirement, the cross-sectional shape of the two wirings 112A and 112B are different from each other at the stepped portion 120, resulting in deterioration of the transmission property of the differential transmission line 113. Specifically, in a position at which they obliquely intersect each other, the structural continuity and symmetry between the wiring 112A (+ electrode) and the wiring 112B (− electrode), which constitute the differential transmission line 113, are broken, as shown in FIG. 10C.
When designing wirings for obtaining an optimal characteristic impedance, the differential transmission line 113 is, in general, designed such that the structure of the constituent two wirings, i.e., the wiring 112A (+ electrode) and the wiring 112B (− electrode) are symmetric to each other as shown in FIG. 10B and FIG. 10D.
In the differential transmission line 113 of FIGS. 10A-10D, since nothing other than differential transmission line 113 is grounded and no capacitance C between the differential transmission line 113 and the ground exists, the capacitance C between the + electrode and the − electrode constituting the differential transmission line will exert a considerable influence on the characteristic impedance. It should be noted that in a case where wirings are provided on both faces of a wiring board, a structure such as shown in FIGS. 10A-10D will be employed in general because the back face of the board cannot be grounded due to restriction in wiring design.
In a configuration as shown in FIG. 10B and FIG. 10D, where a dielectric substance such as an air layer or an insulation resin layer continuously and symmetrically exists between the wiring 112A (+ electrode) and the wiring 112B (− electrode), it is possible to readily calculate the capacitance C between + electrode and − electrode based on the physical property of the materials, the distance between the electrodes, and the like. On the other hand, in a configuration as shown in FIG. 10C, where the structure of the wiring 112A (+ electrode) and the wiring 112B (− electrode) constituting the differential transmission line 113 is discontinuous and asymmetric, characteristic impedance mismatch and so-called intra skew (time difference in transmission between the wiring 112A and the wiring 112B) occurs in the asymmetric section, resulting in turbulence of the waveform of the transmitted signals to deteriorate the signal quality. Particularly, for high-speed transmission of more than 20 GHz, an occurrence of characteristic impedance mismatch and intra skew exert a considerable influence on the transmission property.
As a method of reducing such characteristic impedance mismatch and intra skew, one may consider adjusting the width of the wirings and the thickness of the wirings, but it is difficult to adjust the width of the wirings and the thickness of the wirings in manufacturing.
In other words, since known design methods cannot be practically applied and accurate characteristic impedance cannot be designed, impedance mismatch and intra skew cannot be avoided according to the conventional design. An occurrence of such impedance mismatch and intra skew may not be a practical issue when the frequency is up to about 10 GHz. However, in a wiring board for use in a high-frequency region of 20 GHz or more, slight mismatch of the characteristic impedance and an occurrence of intra skew will exert a considerable influence on the whole transmission path, and thus there is a demand for solving the issues.
The present invention was made in view of the above-described circumstances and an object thereof is providing a wiring board including a differential transmission line showing no difference in cross-sectional shape between two wirings disposed in parallel even in a stepped portion and having an excellent transmission property for high-frequency signals.